47 



Aquarium tanks in August, 1914. On 17th January, 1916, the 

 3^oung fish measured eight centimetres, and had then completed 

 two summers' growths. Fig. 2, Plate II., is a photograph of part 

 of a scale taken from the fish at this date. It is evident that the 

 first mnter is marked on the scale, and from a consideration of 

 the direction of the concentric ridges, especially in the lateral 

 portions of the scale, it is also evident that the marking of the 

 winter is due to a complete stoppage of growth of the scale. 



When the scale is examined under the microscope it is seen 

 that there is at the edge a narrow^ portion which is without con- 

 centric ridges or markings of any kind. The anterior portion 

 of the surface of the scale is thrown into folds which are bounded 

 laterally by radiating grooves and are strongly marked off from 

 the narrow zone already mentioned by a curved line more or less 

 parallel with the edge of the scale. Concentric ridges run across 

 the folds, but at the anterior portion of the fold is a small area in 

 which these ridges are not strongly developed. When growth 

 begins again, the anterior curved boundary of the fold persists, 

 and concentric ridges are formed roughly parallel with those of 

 the previous summer. There is therefore formed by the persist- 

 ence of the anterior boundaries of the folds, which near the long 

 axis of the scale are strongly curved, and by the small area between 

 these boundaries and the last fully developed concentric ridges, 

 a definite marking which serves to indicate the number of winters 

 the fish has lived. 



Laterally, the concentric ridges are not parallel with the edge 

 of the scale, but make a small angle with it. When growth stops 

 in winter these ridges likewise stop, and on growth commencing 

 again, lateral concentric ridges are formed parallel to what was the 

 edge of the scale. Therefore, there is in this region of the scale 

 a definite indication of the stoppage in growth, which will be evident 

 in the photograph. 



When the scale is expressed in terms of the length of the fish, 

 a size of 3-7 cm. is given for the first year's growth. 



The fish was examined 24th May, 1916 ; the length was 8 

 cm., and the scales showed growth had not commenced for this 

 year. 



